Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 307
rigid aluminum tubing and then dropping weighted lengths of #10 or #12 wire from the
ends of each.
The two-element Moxon beam (Fig. 12.8), named for L. A. Moxon, G3XN, who first
described it in 1993, is physically similar to an NVIS horizontal loop wherein the drooping
ends of the two dipoles are pulled up to lie in the same plane as the centers of the
elements but, unlike a loop antenna, the dipole ends are connected together mechanically
but not electrically. Rather than being square, the outline of the Moxon beam as
viewed from above is rectangular because the centers of the two bent dipoles are spaced
less than l/4 to optimize the antenna’s performance as a parasitic array having one
driven element and one reflector. Thus, each of the bent ends of the two elements must
be somewhat less than l/8 long and the center section of each element is perhaps 5l/16
or more in length. An excellent summary of some practical Moxon wire beams for HF
and VHF can be found on DK7ZB’s Web site or at the Moxon Antenna Project site,
www.moxonantennaproject.com.
Like all other two-element Yagis employing a reflector, forward gain, F/B, and SWR
change more rapidly with frequency when approaching the design center of the beam
from lower frequencies. Thus, dimensions and matching circuits should be optimized
for the lower end of the band, since any of these performance characteristics will hold
up respectably well higher up into the band. Of course, a Moxon beam constructed of
tubing will have a wider bandwidth than one using wire.
Top view
INS
A
Reflector
Driven
element
Direction of maximum
gain
INS
D
C
B
E
Figure 12.8 Moxon rectangle.
To cover the entire 20-meter band with a
VSWR less than 1.6:1 into 50– feedline:
A: 296.0''
B: 46.4''
C: 6.0''
D: 56.6''
E: 109.0''